1. Field of the Invention
The present invention relates to an electromagnetic wave absorber for absorbing a microwave effectively and converting it into heat energy and, particularly, to an electromagnetic wave absorber which can be used at high temperatures.
2. Description of the Prior Art
An N type semiconductor material has been available as an electromagnetic wave absorbing material having high microwave absorbing power. This material exhibits a high resistance value at normal temperature but its resistance value sharply drops at high temperatures. Therefore, in an electromagnetic wave absorber made from the above material as a load, the impedance of the load sharply changes along with temperature variations and a microwave cannot be absorbed effectively at a wide temperature range. Electromagnetic wave absorbers which can absorb a microwave effectively even at high temperatures include oxides of metals such as zinc, manganese and cobalt and mixtures of two or more of these metal oxides.
A conventional coated electromagnetic wave absorber can be obtained by coating the above metal oxide on the surface of each barrier of a substrate made from a material which has a honeycomb structure, is essentially composed of alumina, zirconia or the like and rarely absorbs a microwave to form an electromagnetic wave absorbing layer. When this electromagnetic wave absorber is irradiated with a microwave, the microwave is absorbed and converted into heat energy by an metal oxide forming the above electromagnetic wave absorbing layer.
However, the impedance of propagation space determined by the frequency of the propagating microwave (electromagnetic wave) and a medium through which the microwave propagates is not taken into account in the design of the conventional electromagnetic wave absorber. Therefore, the impedance of the electromagnetic wave absorber does not match the impedance of the propagation space. Accordingly, a microwave is reflected upon the surface of the conventional electromagnetic absorber, resulting in a reduction in the absorption efficiency of the microwave. When the metal oxide is coated by a sol-gel process, CVD process or PVD process to form an electromagnetic wave absorbing layer, the impedance of the electromagnetic wave absorbing layer becomes lower than the impedance of the powdery metal oxide as the raw material, and the microwave having a GHz band is greatly reflected. Therefore, the microwave cannot be absorbed efficiently.
When the material forming the above electromagnetic wave absorbing layer contains Co and the material forming the substrate contains Al like a cordierite sintered body essentially composed of MgO or Al.sub.2 O.sub.3, Co and Al react with each other at high temperatures, whereby the composition ratio of the electromagnetic wave absorbing layer differs from the initial composition ratio with the result of a reduction in the electromagnetic wave absorption efficiency of the electromagnetic wave absorber. Also when the material forming the electromagnetic wave absorbing layer contains Mn and the material forming the substrate contains Si like a composite oxide of SiO.sub.2 and MgO, the same reaction occurs with the result of a reduction in the electromagnetic wave absorption efficiency of the electromagnetic wave absorber.